Magnification varying mechanisms of various constructions have been proposed for use in copying machines of different types. With copying machines of the platen scanning optics type wherein an original on the platen is scanned by movable optical elements in the form of a slit and the image of the original is transmitted to a moving photosensitive surface by stationary optical elements including a projection lens, the magnification is varied generally by changing the scanning speed of the movable optical elements in corresponding relation to the desired magnification and shifting the projection lens in a direction along the optical path. When the projection lens is a fixed focal lens, the total conjugate distance varies with the magnification, so that the length of the optical path must be corrected between the original and the photosensitive surface so as to become equal to the total conjugate distance.
For correcting the length of optical path with a variation of magnification, U.S. Pat. No. 3,884,574 discloses an arrangement wherein a first scanner movable at a first speed and a second scanner movable at one half the first speed serve as the movable optical elements, the second scanner being shiftable relative to the first scanner. However, the second scanner, which performs a scanning movement, requires complicated means for shifting.
U.S. Pat. No. 3,947,188 discloses another arrangement for correcting the length of optical path in which the mirror disposed between the original and the projection lens or between the projection lens and the photosensitive drum is a roof mirror comprising two mirror segments and movable along the center or bisecting line of the roof mirror. This mode of shifting the roof mirror has the advantage that the beam incident on the mirror and the beam reflected therefrom remain unchanged in position despite the movement of the mirror. Nevertheless, if such arrangement is used for a transfer copying machine of the platen scanning optics type, there is a need to use at least two mirrors for scanning because an even number of mirrors must be arranged between the original and the photosensitive drum. In the case where these two mirrors individually serve as a first scanner and a second scanner, the optical path becomes non-parallel to the original, entailing the drawback of making the optical system bulky.
Published Examined Japanese Utility Model Application SHO No. 55-52439 discloses still another arrangement in which the second scanner comprises two mirrors disposed at a right angle so that the optical path folded by the first and second scanners will be parallel to the original. For a magnification change, the projection lens is shifted, while a mirror for reflecting the beam through the projection lens toward the photosensitive drum is slidingly moved along the optical axis of the projection lens to correct the length of optical path. The mirror is pivotally movable between a first position and a second position to make beams incident on the photosensitive drum at a definite angle despite changes of magnification. However, with this arrangement in which the angle of incidence of beams on the drum is made definite independently of the magnification, the position where the beam is incident on the drum differs with the magnification circumferentially of the drum. Because the image formed on the drum is transferred to copy paper, the movable optical system and the paper feeder are controlled in synchronism to bring the leading end of the image in register with the leading end of the copy paper, whereas a complex mode of control is required if the position of the incident beam on the drum differs from magnification to magnification.
Moreover, the arrangements disclosed in the above-mentioned prior art references are merely adapted for two- to three-step magnification changes and are therefore unable to meet the recent demand for multistep magnification changes or substantially continuous or stepless magnification changes.